Among secondary batteries, which have recently been increasingly used, demand for rectangular secondary batteries that may be applied to products such as mobile phones and the like due to their small thickness in terms of battery shape is high, and demand for lithium secondary batteries, such as lithium ion batteries, lithium ion polymer batteries, and the like, with high energy density, high discharge voltage, high power output stability, and the like in terms of materials constituting the batteries is high.
Secondary batteries are largely classified into cylindrical batteries, rectangular batteries, and pouch type batteries according to external and internal structural characteristics. Among these, in particular, rectangular batteries and pouch type batteries that may be stacked with a high degree of integration and have a relatively low width to length ratio are receiving attention.
In addition, secondary batteries are also receiving attention as energy sources of electric vehicles, hybrid electric vehicles, and the like, which are alternatives to address air pollution and the like of existing gasoline vehicles, diesel vehicles and the like that use fossil fuels. Thus, applications using secondary batteries are increasingly diversified due to advantages of secondary batteries and application of secondary batteries to more fields and products is anticipated in the future.
As such, as application fields and products of secondary batteries are diversified, batteries are also diversified so as to provide power output and capacity corresponding thereto. In addition, batteries applied to the fields and products strongly require small sizes and lightweight.
For example, small mobile devices such as mobile phones, PDAs, digital cameras, notebook computers, and the like use one to three small and lightweight battery cells per device so as to correspond to the trend of smaller size and lighter weight. By contrast, medium to large-scale devices such as electric vehicles, hybrid electric vehicles and the like use a battery module (also referred to as a “medium to large-scale battery pack”) in which a plurality of battery cells are electrically connected, due to requirements of high power output and high capacity. In this regard, the size and weight of a battery module is directly related to accommodation space, power output, and the like of medium to large-scale devices and thus manufacturers have put much effort into manufacturing relatively small and lightweight battery modules.
A battery module includes a plurality of battery cells and thus an operating state of each battery cell needs to be checked and controlled. For example, it is necessary to secure optimum operating state and safety by detecting physical operating states such as voltage, temperature and the like of each battery cell. Checking of such physical operating states is implemented by a configuration in which a connection member such as wires or the like for measuring voltage is connected to a control unit of each battery cell, which is an object to be measured, or a configuration in which a temperature sensor is installed at an outer surface or in the vicinity of each battery cell, which is an object to be measured, and a detection signal thereof is connected to a control unit of each battery cell using a connection member.
Meanwhile, when a lithium secondary battery is used as a unit cell of a battery module, battery module safety needs to be more carefully considered. Lithium secondary batteries undergo relatively large changes in volume at an anode during intercalation/deintercalation processes. That is, lithium secondary batteries repeatedly expand and contract during repeated charging and discharging and thus internal resistance is increased through such expansion and contraction, which results in significantly reduced battery performance. In addition, a battery case is separated due to gases generated by decomposition of an electrolyte and thus the electrolyte leaks out and fires or explosion may eventually occur. Moreover, combustion or explosion caused in some of the battery cells is continuously transferred to the other battery cells, which may cause severe conditions.
Thus, in consideration of these problems, it may be necessary to check an operating state of each unit cell of a medium to large-scale battery module using a lithium secondary battery as a unit cell, and some related arts disclose a technology of using an LED element as a display unit to display changes in such operating states.
When an LED element is used to display changes in operating states of battery cells, however, a cover of an optical display unit causes optical loss, and light diffusion is not easy due to light straightness of an LED and thus light emitted from the LED element is difficult on the eyes. In addition, when treating an LED element with a separate diffuser film or exposing an LED element by fabrication into a separate lamp, in order for light diffusion of the LED element, additional manufacturing processes are needed, a greater number of components is used, and exterior appearance thereof is not beautiful.
In addition, there is a high need to manufacture a compact, highly structurally stable battery module by considering structural characteristics of a device provided with a battery module installed.
Therefore, there is a very high need to develop technologies that may address these problems and meet such needs.